1. Field of the Invention
The present invention relates generally to sealing systems for effecting a seal between a shaft operating in a closed high pressure system and a housing, and is more particularly concerned with shaft seals employed in extremely high pressure systems such as in a main coolant pump used in a nuclear power plant.
2. Description of the Prior Art
In pressurized water nuclear power plants a reactor coolant is used to transport heat from the reactor core to steam generators for the production of steam. The steam is then used to drive a turbine generator and produce electricity. The reactor coolant system includes a plurality of separate cooling loops, each connected to the reactor core and containing a steam generator and a reactor coolant pump. Systems of this type normally operate at pressures in excess of 1000 psi; in a pressurized water nuclear power plant, the system pressure during operation is significantly in excess of 1000 psi.
A reactor coolant pump typically is a vertical, single stage, centrifugal pump, designed to move large volumes of reactor coolant at high temperatures and pressuresxe2x80x94for example, 550xc2x0 F. and up to 2500 psi. The pump basically includes an intermediate hydraulic shaft seal section located between a lower hydraulic impeller section and an upper motor section. The lower hydraulic section includes an impeller mounted on the lower end of a pump shaft which is operable within a pump casing to pump reactor coolant around the respective loop. The upper motor section includes a motor which is coupled to and drives the pump shaft.
Above the hydraulic section are an internal thermal barrier and external cooling coils which serve to cool system liquid passing therealong. Above the thermal barrier there may be additional clean liquid injected into the housing which is at lower temperature and therefore isolated from the system temperature by the thermal barrier and cooling coils. Sleeve bearings are also provided for the motor and pump shafts, as well as appropriate thrust bearings for the latter, none of which form a part of this invention.
In accordance with the prior art, the intermediate or shaft seal section includes a plurality of vertically separated tandem sealing assemblies, more particularly a primary sealing assembly located at the lower end of the shaft adjacent to and above the pump casing; above the latter is the secondary (back-up) sealing assembly; and above the latter is an upper or tertiary sealing assembly. The sealing assemblies are located concentric to and near the top end of the pump shaft and in a housing which is positioned above the pump impeller. Their combined purpose is to mechanically contain the high positive pressure coolant of the reactor cooling system to prevent leakage along the pump shaft to the reactor containment during both normal and abnormal operating conditions.
The lower, primary sealing assembly is the main seal of the pump. It is typically a hydrostatic, radially tapered, xe2x80x9cfilm-riding,xe2x80x9d controlled-leakage seal, whose primary components are an annular runner which rotates with the pump shaft and a non-rotating annular seal ring which is sealingly mounted to the housing of the lowest seal assembly. The initial design of such a primary seal is described in the afore-mentioned cross-referenced U.S. Pat. No. 347,552 of E. Frisch and has been subsequently modified in its details by, for example, U.S. Pat. No. 3,522,948 of A. N. MacCrum, also referred to above. The primary (or No. 1) seal causes a pressure drop of coolant water from about 2250 psi to 30-50 psi across its face. It allows a flow-rate of 1-3 gallons per minute therethrough. The liquid coolant leaking through the No. 1 seal, now normally at a much lower pressure, flows up the upwardly extending shaft and within the seal housing to a region of the middle, or back-up, sealing assembly. The latter sealing assembly (or No. 2 seal), in accordance with the prior art, has been a rubbing face-type seal. Its primary components have been a rotating runner having an upwardly facing sealing surface and a non-rotating, axially mounted ring located above the runner. During normal operation, this ring and runner provide a rubbing seal. In the unlikely event of No. 1 seal failure, however, the distribution of pressure on the No. 2 seal ring and runner causes them to act as springs and to deflect around their respective centroids, the ring deflecting in a counterclockwise direction and the seal runner deflecting in a clockwise direction, in such a way as to create the converging gap of a hydrostatic, film-riding, face-type seal. In accordance with the prior art (see U.S. Pat. No. 4,961,678, Column 2), as a film-riding face-type seal, the No. 2 seal has the entire high system pressure across it during this emergency condition. During normal operation, however, much of the No. 1 seal leak-off is diverted to a leak-off system. The remaining portion of the coolant passes through the No. 2 seal, leaking at a flow rate of approximately 2 gallons per hour at a pressure across the No. 2 seal of about 30 psi on the higher pressure (inlet) end, which is reduced by the No. 2 seal to 3-7 psi on the lower pressure (outlet) end. The still lower-pressure coolant water leaking through the No. 2 seal flows farther up the shaft and through a region of the upper tertiary (No. 3) sealing assembly.
In accordance with the prior art, the upper or tertiary sealing assembly (or No. 3 seal) has been a rubbing face-type seal, its primary components also being a rotating runner and an axially movable, non-rotating ring. Most of the flow leaking from the No. 2 seal is diverted by the No. 3 seal out through the No. 2 seal leak-off. The rubbing face-type No. 3 seal has been in one of two forms: either it has a double dam seal with two concentric sealing faces, or it has a single dam. The normal, minimal leakage from the No. 3 seal is designed to pass through a No. 3 leak-off system to the containment atmosphere, a situation that reactor systems designers would like to avoid, if possible.
In many nuclear power plants, a material which suppresses neutrons is dissolved into the reactor coolant water. This is normally boric acid enriched with B10 isotope, which acts as the neutron suppressor. The amount of boric acid that can be retained in the reactor system coolant is pressure and temperature dependent. Thus, when the pressure in a reactor system region drops such as occurs across the No. 1, No. 2, and No. 3 seals, the amount of dissolved boric acid in the coolant frequently may not be totally retained in the coolant; thus it may precipitate out into the seal gap to interfere with the effective closing of the seal. To prevent this occurrence, the additional liquid injected into the seal region is intentionally boron-free; because the area of contact between the system liquid and this additional liquid is so small, only a minimal amount of dissolved boron in the system liquid will mix with the added clean liquid.
While it is highly unlikely that there will be a failure of the No. 1 seal, a proposed failure is postulated for safety review purposes by the regulators of nuclear power plants. Thus it is important that back-up systems be provided for this unlikely event. Similarly, it is unlikely that the No. 2 seal could fail either by itself or concurrently with the No. 1 seal; that event, however, is also a postulated safety event and the No. 3 seal is provided to accommodate a breakdown of the No. 2 seal alone. In the event both the No. 1 and No. 2 seals fail, either concurrently or in sequence, while the plant is still under pressure, the No. 3 seal will be required to accommodate full system pressure thereacross. As will be discussed, it is an objective of this invention to provide for a back-up sealing arrangement which can accommodate full system pressure in the highly unlikely event that Nos. 1 and 2 seals fail at the same time, which event may be postulated, nonetheless, by certain regulatory bodies.
Another significant requirement of reactor coolant systems is that the seal systems operate for long periods of time between reactor core refuelings and not be the cause of shutdown of the plant. However, during reactor refueling, seal maintenance is a critical path item. It must be performed with such speed that the down time for seal maintenance is no greater than the time necessary for other maintenance and for reactor core refueling. Thus a reduction of maintenance or change-out time for the primary and back-up seals to ensure that seal maintenance does not increase the critical time path for plant maintenance is an important objective of all seal systems.
A further significant requirement of seal systems is to minimize if not totally prevent the leakage of primary coolant fluid through the seal into the reactor containment, in the unlikely event of seal failure. In accordance with prior art systems, this is accomplished by reliance on a leak-off system adjacent to the back-up sealing arrangement; however, an arrangement which ensures that seal leakage will not occur irrespective of whether or not the pump shaft is rotating at the time of purported failure of the No. 1 or the No. 2 seal, separately or concurrently, is an important objective of this invention.
Another objective is to provide, as the back-up (No. 3) sealing arrangement for normal low-pressure operation, a hydrodynamic, Rayleigh-type face seal upstream of the No. 2 seal, the main back-up seal. A No. 3 seal of the Rayleigh type, which is normally biased closed when the shaft is stationary, is desirable for the sealing system. Furthermore, with low pressure on the upstream side of the No. 3 seal in a main coolant pump where the pump shaft is normally vertically disposed, coupled with possible gas build-up in the sealing gap and with the presence of leak-off systems between the No. 2 and No. 3 seals, there could well result a liquid level below the inlet of a Rayleigh-type No. 3 seal gap or a gas pocket at that inlet. In this event, the Rayleigh-type seal would be running dry and would thereby quickly wear. It is an objective of this invention to provide a Rayleigh-type seal wherein the dry operation thereof during normal operating conditions is significantly reduced.
Field experiences with No. 1 and No. 2 seals have been discussed in Bice et al., U.S. Pat. No. 5,071,315, wherein the use of simple O-rings (together with Teflon inserts) on the secondary sealing face of each of the No. 2 and No. 3 seals may cause problems, particularly if the O-ring moves from its retaining groove and becomes jammed between the seal ring and the adjacent stationary housing part. In that event the axial movement of the seal ring toward and away from its runner surface may be impeded, and a seal ring may well be xe2x80x9chung upxe2x80x9d so that the seal gap does not close the normal axial height. In that case the expected pressure drop across the seal gap may not occur, and the system pressure, or a large part thereof, may be exposed to the high pressure side of the next upstream seal. It is another objective of this invention to reduce the possibility of such occurrences.
The above-mentioned objectives of this invention are accomplished cooperatively and severally in the preferred embodiment by the provision of a new and improved sealing arrangement for the main coolant pumps of a nuclear reactor system. As in the prior art, the primary seal may include a tapered face-type, film-riding, hydrostatic seal ring which breaks down pressure in excess of 1000 psi to approximately 30-50 psi (it being understood that all pressures mentioned in this specification are pressures above atmospheric pressure and therefore gauge pressures). The primary seal ring is movable within limits toward and away from a rotatable primary seal runner surface. The primary seal ring also includes a secondary seal thereon which utilizes the combination of a stationary annular seal housing liner with a generally L-shaped cross section closely received within the seal ring, and an in-line seal such as a normal annular O-ring (with or without a Teflon insert cooperatively mounted therewith) disposed on the seal ring and engaging the housing liner. Thus the only fluid flow path between the primary seal ring and the pressurized housing in which it is located is along the gap between the radially disposed sealing face on the primary seal runner and the complementary sealing face on the axially movable hydrostatic annular sealing ring. It will be noted that in a nuclear power plant the system pressure during normal operation is on the order of 2250 psi.
The invention provides a plurality of back-up seals mounted in fluid series with the primary or No. 1 seal and will be referred to herein as the No. 2, No. 3, and No. 4 seals, all of which perform a back-up function for the No. 1 seal. In accordance with the invention, the No. 2 seal constitutes a face-type annular seal ring which is mounted to move axially toward and away from a sealing surface on a radially disposed seal runner. The No. 2 seal, in accordance with the invention, is a hydrodynamic seal and includes a plurality of Rayleigh-type grooves or pockets mounted in the runner surface, rather than in the ring surface, with the Rayleigh-type pockets being formed in an insert preferably made from silicon nitride, which insert is mounted on the metallic seal runner. The sealing ring is desirably metal and also includes an insert which is normally separated from the seal surface of the runner by a fluid-containing seal gap and is desirably formed of a carbon or carbide compound. The No. 2 seal is provided with a secondary seal assembly which utilizes the combination of an annular liner with a modified L-shaped cross section as part of the housing; however, the secondary seal for the No. 2 seal is a piston-ring arrangement, rather than an O-ring arrangement, which seals between the sealing ring and the housing liner. The hydrodynamic No. 2 seal operates such that a sealing gap is created only when the shaft is rotating; otherwise, when the shaft is stationary, the No. 2 seal gap is forced to be totally closed.
This invention further teaches that the shaft is desirably provided with one or more removable cartridge sleeves mounted thereon located opposite the No. 2, 3, and 4 seals so that they are sealed thereto and held fixed in position. The cartridge sleeve(s) may be removed from the shaft by being lifted over its upper end The seal runners of the No. 2 and No. 3 seals are fixedly mounted on the outer surface of the sleeve(s) such that the removal of the sleeve(s) would include the removal of the entire No. 2, 3, or 4 seal as well as adjacent housing parts by a single lift, or possibly two lifts; and all of this apparatus can be replaced quickly by new (or refurbished) preassembled subassemblies to reduce maintenance time.
Downstream of the No. 2 seal is a No. 3 seal, which is mounted vertically above the No. 2 seal and includes a seal runner mounted on the afore-mentioned cartridge sleeve. The No. 3 sleeve also includes an annular seal ring which is mounted to cooperate with a stationary annular liner housing of the L-shaped cross section and has a sealing face thereon which opposes a radial sealing face on the seal runner. The radial sealing face on the seal ring moves with the seal ring toward and away from the sealing face on the seal runner in the axial direction, forming a seal gap of variable axial size. In this invention the No. 3 seal constitutes a hydrodynamic, Rayleigh-type seal wherein the Rayleigh-type pockets are formed in the seal runner and the seal ring has an insert made of carbon or a carbon-graphite combination which is mounted in the metallic seal ring. The hydrodynamic No. 3 seal is also designed to be normally closed to prevent all leakage along the seal gap when the shaft is stationary. The purpose of the No. 3 seal during normal operations, when the shaft is rotating, is to break down the 30-50 psig pressure on the downstream side of the No. 2 seal to a pressure of approximately 1-3 psig. While the pump system normally has a leak-off on the downstream side of the No. 2 seal below the No. 3 seal, it is possible (1) for the liquid level at the upstream side of the No. 3 seal to be below the seal gap, or (2) for gas pockets at the entrance of the seal gap to be formed, thus causing the No. 3 seal to run dry. In order to avoid this, the Rayleigh-type pockets in the No. 3 seal are formed in the seal runner together with pumping passageways which extend vertically downward from the lower surface of each of the Rayleigh pockets in the seal runner through the runner at a slight angle toward the centerline of the shaft. The passageways will pump into the pockets liquid which is at a level located below the bottom of the Rayleigh-type pockets in the seal runner, but above the lower end of the pumping passageways, thereby ensuring that the No. 3 seal runs with liquid in the pockets. Also, a gas drain may be formed in the No. 3 seal ring adjacent the seal gap inlet to prevent gas pocket formation. The No. 2 seal ring also has a gas drain formed on it in the same location as on the No. 3 seal.
This invention also provides a new back-up seal referred to herein as the No. 4 seal, located vertically above the No. 3 seal, which serves as a back-up and shutdown seal capable of preventing leakage of liquid (or even a mist) from the interior to the exterior of the pump casing irrespective of the pressure across the Nos. 1 to 4 seals and irrespective of the rotation of the pump shaft. This is accomplished by forming the No. 4 seal as a segmented ring seal which has its primary sealing surface co-acting with a vertically extending surface on the pump shaft. In this example the sealing surface on the pump shaft is formed on a cartridge sleeve which is sealingly mounted on the pump shaft to prevent leakage therebetween. The No. 4 seal, being a segmented type, during normal operation may comprise a rubbing seal having hydrodynamic means thereon for producing negative hydrodynamic lift in a sealing surface of the seal ring segments, in the event liquid reaches the No. 4 seal at higher than normal pressure or even with full system pressure across it For example, a seal of the type described in U.S. Pat. No. 4,082,296, issued to Philip C. Stein, serves well in this application. It will be noted that, in the unlikely event of failure of both the No. 1 and the No. 2 seals, total system pressure would appear across the No. 3 seal, which is formed to be completely closed when the shaft is stationary. Should the shaft be rotating at the time of the proposed failure, total system pressure would be across the No. 3 seal, which could not be totally broken down by the No. 3 seal even though some pressure breakdown would be achieved, and thus a good-sized portion of the full system pressure would extend to the inlet of and across the No. 4 seal. If very high system pressure appears across the No. 4 seal, it will serve to force the seal segments into contact with both the shaft sleeve sealing surface and the adjacent housing sealing surface, irrespective of whether or not the shaft is rotating, and thereby (for a significant length of time during which the reactor system will be shut down) prevent leakage from passing out of the interior of the pump housing.
These and other features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings, wherein is shown and described an illustrative embodiment of this invention, it being understood that each of the improvements may be used separately without use of the other improvements, and such use remains within the contemplation of this invention.